9378

• UCREFRP Catalog Number: 9378
• Author: Nesler, T. P.
• Title: Recovery of the Colorado River Endangered Fishes
• USFW Year: 2000.
• USFW - Doc Type: Biological recovery goals and criteria for Colorado pikeminnow, Humpback chub, Razorback sucker & Bonytail.
• Copyright Material: NO

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increasing (White, Green) over the 6-7 year estimation period (Figures 1-4). The
yearly changes are not likely statistically significant for any of these rivers given
overlapping 95% confidence intervals ranging from 21-150% of the point estimate
(Table A-1). The larger populations appear in the Green (2,000-7,400 approx.)
and the Colorado rivers (900-1,200 approx.). Numbers of Colorado pikeminnow
per river mile range from 4.8-8.6 on the Colorado River and 6.1-22.2 on the Green
River. For the tributary rivers, the White and Yampa, Colorado pikeminnow
populations appear similar in size, and significantly smaller in size and density
compared to the Green and Colorado rivers. Tributary fish population numbers
range from 180-550 fish and 1.8-5.4 fish/mile on the White River and 120-260 fish
2.3-5.0 fish/mile on the Yampa. For the Green and Colorado rivers, the ratio of
recaptured/captured fish increased over the 1991-97 estimation period, while the
ratios for the White and Yampa rivers varied with no trend apparent. A significant
attribute of the Yampa population estimation is the exclusion of the Yampa Canyon
reach, comprising the lower 45 river miles. This was necessary since the mark-
recapture data available from that reach was collected almost exclusively during
the migration-spawning period, which involves adult fish from the Green, White
and Yampa rivers. The presence and size of an adult pikeminnow population in
the lower Yampa is uncertain.

The only comparison of population abundance available comes from
Osmundson and Burnham (1996) and unpublished data in Osmundson (1999, in
review). For the Colorado River population of pikeminnow, their estimates (and
95% confidence intervals) were 1992-535 fish (320-1594), 1993-675 fish (464-
1367), and 1994-629 fish (393-1245). These are 39-47% lower than the estimates
derived here. In 1998-99, Osmundson (2000, in press) estimated the Colorado
River population at 730-800 fish, suggestive of further increases in population
abundance through the 1990s. The authors did not provide 95% confidence
intervals for the combined abundance estimates for the upper and lower reaches,
so they were approximated using the coefficients of variation evident from the
available standard errors and confidence intervals. These interval estimates
overlap with those calculated in Table A-1, and the upper confidence intervals
estimated from Osmundson and Burnham (1996) include the point estimates here.
While the 1992-94 estimates here suggest a steady increase, those of Osmundson
and Burnham (1996) peak in 1993.

Catch per unit effort data for the Yampa, Colorado, White and Green rivers
all indicate steady increases in Colorado pikeminnow abundance since 1991
(McAda et al. 1998)

Length frequency distributions for these Colorado pikeminnow populations
(Figures 5-8, Appendix Table A-3) reflect stable size structures with the influence
of variable year class strength apparent in the number, magnitude, and location of
the peaks. Attributes of stability are the consistent presence of both small and
large fish and modal peaks occurring mid-range. No continual or persistent shift of
the frequency distribution to the right (lack of recruitment) or left (selective mortality